Disentangling homogenization, pseudohomogenization, and cryptic homogenization: Application of a null model for invasive species’ effects on similarity

Background/Question/Methods

One potential consequence of the widespread introduction of nonindigenous species is the loss of regional biotic distinctiveness, leading to taxonomic homogenization of biotas among localities.  Similarity indices such as Jaccard’s Index are often used to assess changes in compositional similarity resulting from species introductions.  These indices are therefore useful metrics of taxonomic homogenization in invaded regions.  However, changes in species richness alone can result in significant changes in similarity for probabilistic reasons, leading to potential misinterpretations of the role of introduced species in biotic homogenization.  A null model approach can be used to account for this probabilistic effect of species richness on similarity measures, allowing a more accurate assessment of the effects of nonindigenous species on biotic similarity and biodiversity. 
Results/Conclusions

Our null model results show that similarity indices can suggest biotic homogenization when in fact none has occurred (“pseudohomogenization”), or they can suggest an absence of homogenization when it actually has occurred (“cryptic homogenization”).  Null expectations can also be generated to predict situations where misinterpretations are most likely.  For example, misinterpretation become more likely when a relatively large number of minimally-invasive species or when relatively few, highly-invasive species become established within a region.  We suggest that the more widespread application of a null model approach to studies of biotic homogenization will improve our understanding of how species introductions affect compositional similarity and regional diversity.